Civil Engineering Reference
In-Depth Information
19
Prestressed Concrete
19.1
INTRODUCTION
Prestressing can be defined as the imposition of internal stresses into a structure that are of
opposite character to those that will be caused by the service or working loads. A rather
common method used to describe prestressing is shown in Figure 19.1, where a row of
topics has been squeezed together by a person's hands. The resulting “beam” can carry a
downward load as long as the compressive stress due to squeezing at the bottom of the
“beam” is greater than the tensile stress there due to the moment produced by the weight of
the topics and the superimposed loads. Such a beam has no tensile strength and thus no mo-
ment resistance until it is squeezed together or prestressed. You might very logically now
expand your thoughts to a beam consisting of a row of concrete blocks squeezed together
and then to a plain concrete beam with its negligible tensile strength similarly prestressed.
The theory of prestressing is quite simple and has been used for many years in vari-
ous kinds of structures. For instance, wooden barrels have long been made by putting
tightened metal bands around them, thus compressing the staves together and making a
tight container with resistance to the outward pressures of the enclosed liquids. Prestress-
ing is primarily used for concrete beams to counteract tension stresses caused by the
weight of the members and the superimposed loads. Should these loads cause a positive
moment in a beam, it is possible by prestressing to introduce a negative moment that can
counteract part or all of the positive moment. An ordinary beam has to have sufficient
strength to support itself as well as the other loads, but it is possible with prestressing to
produce a negative loading that will eliminate the effect of the beam's weight, thus pro-
ducing a “weightless beam.”
From the preceding discussion it is easy to see why prestressing has captured the imagi-
nation of so many persons and why it has all sorts of possibilities now and in the future.
In the earlier chapters of this topic, only a portion of the cross sections of members in
bending could be considered effective in resisting loads because a large part of those cross
sections were in tension and thus cracked. If, however, flexural members can be pre-
stressed so that their entire cross sections are kept in compression, then the properties of
the entire sections are available to resist the applied forces.
For a more detailed illustration of prestressing, reference is made to Figure 19.2. It is
assumed that the following steps have been taken with regard to this beam:
1.
Steel strands (represented by the dashed lines) were placed in the lower part of the
beam form.
2.
The strands were tensioned to a very high stress.
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